The present invention relates to optical transmission modules for optical communication and, in particular, to an optical transmission module for use in the transmitter sections of 10 Gbits/s high transmission rate optical transceivers.
The semiconductor laser-used optical transmission module is one of the key devices of transceivers for optical fiber transmission. With the spread of broadband networks, optical transmission modules have recently so advanced in transmission speed that the bit rate of 10 Gbits/s is common. Optical transmission modules in this field of application are required to not only reduce the size, power consumption and cost but also realize good transmission waveform quality.
A prior art optical transmission module is discussed in, for example, Japanese Patent Laid-open No. 2002-374028. FIG. 6 shows a sub-mount plate having a semiconductor laser mounted thereon in the prior art optical transmission module. Its top view and front view are shown respectively in FIG. 6A and 6B. Reference numeral 1 denotes a semiconductor laser diode, 2 is an insulation plate made of ceramics, 3 is a ground electrode, 4 is a signal transmission line, 5 is a resistor element, 6 is a thin-film inductor, 7 is a bottom ground electrode and 8 is a reduced thickness formed below the thin film inductor. In this prior art example, the direct modulation type semiconductor laser diode 1 is mounted adjacent to the thin film inductor 6 on the same insulation plate 2 having the thin film inductor 6 mounted thereon and a bias current is supplied to the laser diode 1 via the thin film inductor 6. This configuration is intended to reduce the bonding wire inductance and the capacitance parasitic to the inductor so as to minimize the adverse effect on the high frequency signal. The signal transmission line 4 is formed as a grounded coplanar waveguide type transmission line having a ground electrode line formed along each side thereof and a bottom electrode 7 formed on the bottom of the plate 2. Usually, a voltage drive type driver IC is used externally. The series resistance of the resistor element 5 and semiconductor laser diode 1 and the characteristic impedance of the signal transmission line are matched to the output impedance (for example 50 Ω) of the driver IC so as to transfer the voltage signal of the driver IC to the semiconductor laser diode 1 without causing reflection of the voltage signal.